Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch early measurements

Jarkko Sakkinen
2024-04-04T14:55:29Z

On Thu Apr 4, 2024 at 2:56 AM EEST, Eric Biggers wrote:
> On Wed, Apr 03, 2024 at 09:32:02AM -0700, Andy Lutomirski wrote:
> > On Fri, Feb 23, 2024, at 10:30 AM, Eric Biggers wrote:
> > > On Fri, Feb 23, 2024 at 06:20:27PM +0000, Andrew Cooper wrote:
> > >> On 23/02/2024 5:54 pm, Eric Biggers wrote:
> > >> > On Fri, Feb 23, 2024 at 04:42:11PM +0000, Andrew Cooper wrote:
> > >> >> Yes, and I agree.  We're not looking to try and force this in with
> > >> >> underhand tactics.
> > >> >>
> > >> >> But a blind "nack to any SHA-1" is similarly damaging in the opposite
> > >> >> direction.
> > >> >>
> > >> > Well, reviewers have said they'd prefer that SHA-1 not be included and given
> > >> > some thoughtful reasons for that. But also they've given suggestions on how to
> > >> > make the SHA-1 support more palatable, such as splitting it into a separate
> > >> > patch and giving it a proper justification.
> > >> >
> > >> > All suggestions have been ignored.
> > >>
> > >> The public record demonstrates otherwise.
> > >>
> > >> But are you saying that you'd be happy if the commit message read
> > >> something more like:
> > >>
> > >> ---8<---
> > >> For better or worse, Secure Launch needs SHA-1 and SHA-256.
> > >>
> > >> The choice of hashes used lie with the platform firmware, not with
> > >> software, and is often outside of the users control.
> > >>
> > >> Even if we'd prefer to use SHA-256-only, if firmware elected to start us
> > >> with the SHA-1 and SHA-256 backs active, we still need SHA-1 to parse
> > >> the TPM event log thus far, and deliberately cap the SHA-1 PCRs in order
> > >> to safely use SHA-256 for everything else.
> > >> ---
> > >
> > > Please take some time to read through the comments that reviewers have left on
> > > previous versions of the patchset.
> >
> > So I went and read through the old comments, and I'm lost. In brief summary:
> >
> > If the hardware+firmware only supports SHA-1, then some reviewers would prefer
> > Linux not to support DRTM. I personally think this is a bit silly, but it's
> > not entirely unreasonable. Maybe it should be a config option?
> >
> > If the hardware+firmware does support SHA-256, then it sounds (to me, reading
> > this -- I haven't dug into the right spec pages) that, for optimal security,
> > something still needs to effectively turn SHA-1 *off* at runtime by capping
> > the event log properly. And that requires computing a SHA-1 hash. And, to be
> > clear, (a) this is only on systems that already support SHA-256 and that we
> > should support and (b) *not* doing so leaves us potentially more vulnerable to
> > SHA-1 attacks than doing so. And no SHA-256-supporting tooling will actually
> > be compromised by a SHA-1 compromise if we cap the event log.
> >
> > So is there a way forward? Just saying "read through the comments" seems like
> > a dead end.
> >
>
> It seems there may be a justification for some form of SHA-1 support in this
> feature. As I've said, the problem is that it's not explained in the patchset
> itself. Rather, it just talks about "SHA" and pretends like SHA-1 and SHA-2 are
> basically the same. In fact, SHA-1 differs drastically from SHA-2 in terms of
> security. SHA-1 support should be added in a separate patch, with a clearly
> explained rationale *in the patch itself* for the SHA-1 support *specifically*.

Yeah, this is important so that we don't end up deleting that support
by accident. Just adding to denote that this more than just a "process
issue".

> - Eric

BR, Jarkko